1. Technical Field
The present invention relates to an optical circuit board including a reflection surface for redirecting the optical signal and a method for producing the same.
2. Background
The conventional optical circuit board will be described with reference to FIG. 3. The conventional optical circuit board B includes a wiring board 20, and an optical waveguide forming portion 21 as shown in FIG. 3.
The wiring board 20 includes a lower insulating layer 22a and an upper insulating layer 22b. A connection hole 23 is formed in each of the insulating layers 22a and 22b. A lower-layer wiring conductor 24a is formed on the upper surface of the lower insulating layer 22a and in the connection hole 23. An upper-layer wiring conductor 24b is formed on the upper surface of the upper insulating layer 22b and in the connection hole 23. An external connection pad 25 is formed on the lower surface of the lower insulating layer 22a. 
The optical waveguide forming portion 21 is formed on the wiring board 20. An optical waveguide 26 and an electronic component connection pad 27 are formed in the optical waveguide forming portion 21.
The optical waveguide 26 is formed by a lower clad layer 26a, a core 26b, and an upper clad layer 26c. An optical signal is transmitted to the optical waveguide 26.
The electronic component connection pad 27 is formed on the upper surface of the optical waveguide 26 (upper clad layer 26c). An electronic component (not shown) is mounted on the electronic component connection pad 27. Optical signals are transferred between the electronic component and the optical waveguide 26.
A connection hole 28 is formed so as to penetrate the optical waveguide forming portion 21 in the optical waveguide forming portion 21. A connection conductor 28a is deposited in the connection hole 28. The electronic component connection pad 27 and the upper-layer wiring conductor 24b are connected through the connection conductor 28a. 
The lower clad layer 26a and the upper clad layer 26c constituting the optical waveguide forming portion 21 are solid insulating layers. The core 26b is a thin strip having a rectangular cross-section. The lower clad layer 26a and the upper clad layer 26c are in close contact with the surface of the core 26b, and surround the core 26b. 
The core 26b has a reflection surface M at one end. The reflection surface M includes an inner surface of a groove being perpendicular to the extending direction of the core 26b in a plane view and having a predetermined angle with respect to the upper surface in a cross sectional view. The transfer of optical signals is performed between the optical waveguide 26 and the electronic component through the reflection surface M.
Next, the method for producing the conventional optical circuit board will be described with reference to the principal part enlarged views shown in FIGS. 4A to 4I. The same members as in FIG. 3 will be denoted by the same reference numerals and will be described.
First, as shown in FIG. 4A, the lower insulating layer 22a on the upper surface of which the lower-layer wiring conductor 24a is deposited, and on the lower surface of which the external connection pad 25 is deposited is prepared.
Next, as shown in FIG. 4B, the upper insulating layer 22b is laminated on the upper surface of the lower insulating layer 22a, and the connection hole 23 having the lower-layer wiring conductor 24a as the bottom surface is formed.
Next, as shown in FIG. 4C, the upper-layer wiring conductor 24b is deposited on the upper surface of the upper insulating layer 22b and in the connection hole 23, whereby the wiring board 20 is formed.
Next, as shown in FIG. 4D, the lower clad layer 26a is formed on the upper surface of the wiring board 20.
Next, as shown in FIG. 4E, the core 26b is formed on the upper surface of the lower clad layer 26a. 
Next, as shown in FIG. 4F, the upper clad layer 26c is formed on the upper surface of the core 26b, whereby the optical waveguide 26 is formed.
Next, as shown in FIG. 4G, the connection hole 28 exposing a part of the upper-layer wiring conductor 24b as the bottom surface is formed in the optical waveguide 26.
Next, as shown in FIG. 4H, the connection conductor 28a is deposited in the connection hole 28, and the electronic component connection pad 27 is formed on the upper surface of the upper clad layer 26c. 
Lastly, as shown in FIG. 4I, a laser beam is irradiated obliquely from above the optical waveguide 26 to divide the core 26b, and the reflection surface M including an inner surface of the groove being perpendicular to the extending direction of the core 26b and having a predetermined angle with respect to the upper surface of the wiring board 20 is formed, whereby the conventional optical circuit board B is formed as shown in FIG. 3.
For example, Japanese Unexamined Patent Application Publication No. 2006-330066 discloses a high refractive index core layer having a cross-section of substantially rectangular shape, a low refractive index upper clad layer covering the high refractive index core layer, and a groove forming the optical reflection surface for propagating the optical signal, propagating through the high refractive index core layer, through the high refractive index core layer by bending the optical signal into an upward or downward direction at a right angle, so that the optical signal propagating through the core layer can be bent at a right angle. The CO2 laser beam is irradiated at the high refractive index core layer from above the low refractive index upper clad layer, and therefore a groove having an inclination of approximately 45° with respect to the high refractive index core layer and being at least wider than the width of the high refractive index core layer is formed. The optical reflection surface is constituted by the flat portion of the groove wall.
By the way, in recent years, as the miniaturization and high functionality of electronic devices typified by portable communication devices and music players is advanced, the optical circuit boards mounted on these electronic devices are also required in miniaturization and high functionality. For this reason, for the wiring conductor of the wiring board constituting the optical circuit board, a fine wiring conductor is made so as to be formed in high density.
However, when the reflection surface M is formed by the method for producing the conventional optical circuit board B, there are cases where the laser beam irradiated obliquely from above penetrates the upper insulating layer 22b and reaches the fine lower-layer wiring conductor 24a formed in high density to damage. As a result, there is a problem that the electrical signal does not easily propagate to the lower-layer wiring conductor 24a, and that the electronic component does not operate stably.